Work vanilla into the sugar, using a pastry blender or clean hands, until well distributed. Sift the cocoa into a separate, very large bowl; blend in the salt and milk powder. Gradually stir the sugar mixture into the cocoa until well mixed. The color should be uniform throughout. Store in an airtight container. To make one serving of cocoa place 1/2-cup of mix in a large mug, gradually add one-cup very hot (180-200ºF) water while stirring to dissolve mix. Top with marshmallows or whipped cream if desired.